1. Field
This disclosure relates to the construction of compact high energy fiber laser pulse sources, methods of designing and using such sources, and adaptive control techniques therefore.
2. Description of the Related Art
Over the last several years, fiber lasers and amplifiers have been regarded as the most promising candidates for pulse sources for industrial applications, due to their unique simplicity of construction. Large core fiber amplifiers and specifically large core diffraction limited multi-mode amplifiers (as described by M. E. Fermann and D. Harter, ‘Single-Mode Amplifiers and Compressors based on Multi-Mode Optical Fibers,’ U.S. Pat. No. 5,818,630) enable the amplification of optical signals to levels where micro-machining and laser marking become possible. See A. Galvanauskas, et al., U.S. patent application Ser. No. 09/317,221. Since laser marking and micro-machining are dependent on the supply of high peak power pulses, it is advantageous to use such fiber amplifiers for the amplification of nanosecond regime (ns) pulses, as supplied, for example, by micro-chip lasers or general Q-switched sources. In conjunction with large-core fiber amplifiers, these ns pulse sources can be amplified to pulse energies in the hundreds of microjoules (μJ). Such fiber amplifier systems can operate as direct replacements for Nd-based solid state lasers in any micro-machining and marking application.
The use of diffraction limited fiber multi-mode amplifiers allows a significant improvement in power density to be delivered to a target compared to non-diffraction-limited multi-mode amplifiers as described for example in B. Desthieux, Appl. Phys. Lett., vol. 63, No. 5, pp. 586-588 (1993). Note, that in this early work the use of electrically driven semiconductor lasers for the generation of short optical seed pulses to high power amplifier chains, albeit with a final multi-mode power amplifier, was already described.
The use of electrically driven semiconductor lasers for amplifier chains was later reiterated by Grubb et al., U.S. Pat. No. 6,151,338. A variety of complex fiber amplifier implementations for use in micro-machining applications again reiterating the use of electrically driven semiconductor seed lasers was recently also suggested in U.S. Pat. No. 6,433,306 to Grubb et al. In another suggestion by Grubb et al., U.S. Pat. No. 5,892,615, the use of bent single-mode amplifier fibers has been suggested; the difficulty in making such single-mode amplifiers has also been a limitation in such high power fiber amplifier systems.
The most advanced micro-machining or micro-structuring applications are enabled by amplification of ultrafast optical pulses in fiber media. Ultrafast optical pulses are generally characterized by a pulse width of less than 50 ps; conveniently, chirped pulse amplification is implemented to enable the amplification of such pulses to the μJ-mJ energy range. Generally, chirped pulse amplification systems use a near bandwidth-limited seed pulse source, which is temporally stretched (i.e. chirped) in a pulse stretcher before amplification in a power amplifier. After amplification, the pulses are recompressed to approximately the bandwidth limit using a pulse compressor.
Commercially viable fiber chirped pulse amplification systems were suggested in A. Galvanauskas and M. E. Fermann, ‘Optical Pulse Amplification using Chirped Bragg Gratings,’ U.S. Pat. No. 5,499,134. The system in this work relied on chirped fiber Bragg gratings for pulse stretching. Chirped fiber Bragg gratings have been developed into widely available commercial devices. The chirp imparted by the Bragg gratings can be designed to be linear or nonlinear to compensate for any order of dispersion in a chirped pulse amplification system. See A. Galvanauskas et al., ‘Hybrid Short-Pulse Amplifiers with Phase-Mismatch Compensated Pulse Stretchers and Compressors,’ U.S. Pat. No. 5,847,863, which is important for the generation of near bandwidth limited pulses after pulse recompression.
Generally, in such systems, as a compromise between system compactness and high-energy capability, the use of a chirped fiber Bragg grating pulse stretcher in conjunction with a bulk grating pulse compressor is advantageous, providing at least partial integration of the high-energy fiber amplifier system. Alternative arrangements resorting to the use of bulk stretchers and compressors (as generally used in the state of the art) are generally much more difficult to align, require a significantly larger amount of space for their operation and are only of limited utility in real industrial applications.
However, to date, the mismatch in the dispersion profile between fiber grating pulse stretchers and bulk grating pulse compressors has limited the compressibility of the pulses, limiting their acceptance in the field of ultrafast optics.
Recently, modular, widely tunable fiber chirped pulse amplification systems were described by M. E. Fermann et al., ‘Modular, High Energy Widely Tunable Ultrafast Fiber Source’, U.S. patent application Ser. No. 09/576,772, that further enhance the utility of such fiber laser sources in industrial applications.
What remains to be demonstrated is a particularly cost-effective, easily manufacturable and flexible fiber chirped pulse amplification system that is practical for mass production, but yet exhibits high pulse stretching and compression ratios by benefit of matched dispersion profiles.